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Full text of "The manufacture and properties of iron and steel"

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STEEL CASTINGS.                                        415
third was hot-forged from a star-shaped section of about 2 inches by 1$ inches into a bar 1J inches by three-eighths inch, and after being cooled was twisted into a closed corkscrew. Similar pieces were exhibited by Krnpp in his magnificent exhibit at.Chicago, but we stand ready in America to duplicate any such metal on regular contracts.
Such trials, made on castings taken at random, are preferable to tensile tests from sample bars, since the small pieces will not be in the same physical condition as the larger castings. A flaw or blowhole in the small test does not imply that the casting contains similar imperfections, and while an open cavity which is visible on the surface of a machined test will have a disastrous effect upon the strength and ductility, it might be of slight importance if buried in the interior. This necessity of having a perfect surface it difficult to conduct a series of tests with the same dimension of test-pieces, for if five-eighths inch in diameter is the desired size, it may be necessary to turn some of the pieces to one-half inch, while the length must sometimes be reduced to 6 or 4 inches. It is also an argument against an 8-inch test piece, for the chance of pinholes and a consequent bad record is thereby multiplied fourfold.
This test piece should not be annealed unless the castings themselves are to be treated in the same manner, and although it is customary to anneal most structural work, it is not necessary in many cases if the best of stock is used. This will be called heretical by many engineers, but the tests just recorded upon an unannealed gear-wheel will show that the metal can be exceptionally tough in its original state. In castings of complicated shape and exposed to shock, annealing should be specified, but it must be remembered that there is no magic charm in this word. It is not sufficient to say that they shall be annealed and make sure only that they are covered with soot or fresh oxide. The heat treatment of steel is a scientific procedure, by which the metal is raised to an accurately determined critical temperature, whereby certain molecular rearrangements occur. If these rearrangements are properly guided, the result will be a fine-grained structure and a tough metal. If not properly guided the last condition may be as bad as the first.
IJp to within a few years most steel castings were made of hard metal containing from .30 to .50 per cent, of carbon, and having a